1. Technical Field
The present disclosure relates to a fuel assembly, and in particular, relates to a fuel assembly having a particular fuel rod arrangement.
2. Description of the Related Art
In general, a fuel assembly for a light water reactor is formed by bundling a plurality of fuel rods having a number of fuel pellets, such as uranium oxide filled into fuel canning pipes, and one or several pieces of water rods parallel to each other with a spacer and surrounding the outer circumference with a channel box.
In the related art, a fuel assembly in which fuel rods are arranged to be square-shaped has been widely used.
FIG. 20 is a plane sectional view of a fuel assembly having fuel rod arrangements that is square grid-shaped in the related art. Numeral 20 indicates the entire fuel assembly. Numerals 21, 22 and 23 respectively indicate a fuel rod, a channel box and a water rod.
As illustrated in FIG. 20, the fuel assembly 20 includes a fuel rod arrangement of eleven rows by eleven columns. The fuel rods 21 are provided at apexes of regular tetragons arranged to be square grid-shaped. The water rod 23 is arranged on a diagonal line from the corner of the channel box 22 that is closest to a control rod (not illustrated) to the opposing corner.
Further, in the related art, there has also been proposed a fuel assembly that includes a fuel rod arrangement having the mentioned square grid that is rotated by 45 degrees against one side of the channel box.
Recently, demand for enhancing efficiency of a reactor has been strengthened. To address this issue, from a viewpoint that fuel economy can be improved by further enlarging energy extracting from each fuel assembly, it has been attempted to load a larger number of fuel rods into one fuel assembly.
Further, also from a viewpoint of waste amount reduction, it is preferable that the number of fuel rods accommodated by one fuel assembly is large in order to reduce the waste amount.
Accordingly, when designing a fuel assembly for a boiling-water reactor (BWR) in the related art, there has been a tendency that the number of fuel rods accommodated in the channel box is increased to be eight rows by eight columns, nine rows by nine columns and ten rows by ten columns (see Japanese Patent Application Laid-open (JP-A) No. 2009-145203).
Regarding the arrangement of fuel rods, arranging fuel rods to be square grid-shaped constituted with rows and columns parallel to sides of a channel box or arranging fuel rods is a conventional fuel assembly. Further, a conventional fuel assembly that rotates the square grid against one side of a channel box by 45 degrees has been a basic arrangement in the related art.
Both of the above fuel rod arrangements are fuel rod arrangements of a square grid. It may be generally perceived that almost every practical fuel rod arrangement has been a fuel rod arrangement of a square grid in the related art. It has been favorable with the fuel rod arrangement having the square grid-shaped arrangement that is parallel to the sides of the channel box against an insertion position of a control rod or the fuel rod arrangement having the square grid rotated against one side of the channel box by 45 degrees because of the way that the fuel assembly advances in the reactor for combustion. With respect to combustion, the fuel assembly advances symmetrically against a diagonal line connecting a corner of the channel box that is close to the center of the control rod and a diagonal corner opposing thereto (hereinafter, merely called the diagonal line when apparent) in relation to the control rod. Here, diagonal symmetry of fuel denotes that fuel is arranged symmetrically against the diagonal line of the channel box as described herein. In the case that the diagonal symmetry of fuel is satisfied, an even burnup can be actualized within the fuel assembly, so that soundness and economy of fuel can be further improved.
However, with the fuel rod arrangement that is square grid-shaped, the distance between the fuel rods located on adjacent apexes of the regular tetragon cannot be lessened than a determined distance due to soundness, while the distance between the fuel rods located on the diagonal line is inevitably longer than the distance between the fuel rods at the adjacent apexes. Accordingly, there has been a limitation of the number of fuel rods that can be accommodated by one fuel assembly.
Meanwhile, as disclosed in JP-A No. 1992-143694, a fuel rod arrangement having all fuel rods provided at apexes of regular triangles has been proposed solely for the purpose of increasing the number of fuel rods in a fuel assembly.
With the fuel rod arrangement having fuel rods arranged to be regular triangle grid-shaped, the respective distances between all of the fuel rods is equalized and each distance between the fuel rods can be shortened. Accordingly, the fuel rods can be arranged at a higher density.
However, as can be seen from the drawings of JP-A No. 1992-143694, the fuel rod arrangement that is regular triangle grid-shaped is not a symmetric arrangement against the diagonal line of the channel box. That is, when the fuel rods are arranged in a triangle grid-shaped arrangement, absolute diagonal symmetry of fuel cannot be actualized.
Further, as shown in the related art of FIG. 20, a water rod 23 has been provided into a fuel assembly for the purpose of planarization of output distribution of the fuel assembly, improvement of the reactivity, and improvement of the void reactivity coefficient.